Color Ramp vs RGB Curves in Cycles
They are different tools for doing the same things in different ways.

Color Ramp


Easier to precisely set colors and stops
Harder to make custom falloffs/interpolations


RGB Curves


Easier to make your own falloffs/distributions
Harder to specify exact colors


As an example, I attempted to replicate the result of an RGB curve setup with a color ramp node. Note the differences around the transitions due to the color ramp interpolation:


I used the compositor for this, but they work the same in Cycles



Which you use depends largely on what you want to do and what your preference is. I tend to use color ramps for precision stuff, and curves for more artistic things.